In the borehole environment, lithology identification is usually made by cross plotting the responses of the standard gamma ray bulk density tool and the standard neutron porosity tool, possibly aided by information of the photoelectric factor (P.sub.e) of the formation and gamma ray information from the standard gamma ray log. Although these techniques work reasonably well for resolving lithology issues in simple formations or binary mixtures, they are not always adequate in more complex lithologies involving mixtures of different minerals, such as limestone formations with variable inclusion of dolomite and anhydride, sandstones with calcite cementing, or calcium-silicate formations. Moreover, the standard bulk density log utilizes a cesium 137 gamma ray source. As this isotope is highly radioactive and relatively long-lived (30 year half-life), it represents a potential hazard both to operating personnel and to the environment in the event of loss during use.
Efforts have been made heretofore to overcome the limitations of the aforementioned cross plot techniques, which in the main rely on tool responses primarily sensitive to porosity, by developing lithological information based on the elemental analysis of formations using neutron-induced gamma ray spectroscopy. In general, logging tools measuring neutron-induced spectra employ a 14 MeV D-T neutron accelerator as the neutron source and select the type of gamma ray to be detected, i.e., inelastic scattering, thermal neutron capture or activation, by appropriate timing of the detector gates. The measured spectra are then processed to develop the elemental information. In one form of data processing, a spectral fitting technique compares a measured spectrum to a linear sum of weighted standard spectra. The weight applied to each of the standards is varied until the sum is, in a least-squares sense, the best fit to the observed spectrum. The weights (yields) then represent the relative contributions to the total spectrum of the elements in the standard set.
Use of the aforementioned spectral-fitting method to analyze thermal neutron capture spectra for lithology purposes can provide much useful information at relatively good logging speeds, as described, for example, by Westaway et al. in "The Gamma Spectrometer Tool Inelastic and Capture Gamma-Ray Spectroscopy For Reservoir Analysis" Soc Pet Eng 55th Ann Fall Conf., Sept., 1980, Paper SPE-9461. The determination of absolute elemental yields from the relative elemental yields measured by the logging tool, however, has always been a difficult problem. This is caused by several factors, the most important of which is that the relative yields, from capture, are extremely sensitive to borehole and formation sigmas (macroscopic capture cross section). Because of such sigma sensitivity, the depth of investigation of the capture measurement can vary substantially from borehole to borehole (or even within a single borehole as borehole and/or formation conditions change), e.g., from 6 to 30 inches, making environmental correction of the measurement difficult. Also, as the salinity of the borehole or formation fluids increases, not only does the depth of investigation shrink, but the relative contribution of the lithologically important gamma rays decreases as chlorine-originated capture gamma rays increase. In fact, in a highly saline borehole/formation environment chlorine gamma rays can amount to as much as 90% of the signal. This additional factor can seriously degrade the results of the capture spectral fitting analysis.
The aforementioned Westaway et al. Paper SPE-9461 also describes the analysis by spectral fitting of observed inelastic scattering gamma ray spectra to obtain relative elemental yields for carbon and oxygen, which are used to derive a C/O estimate, and calcium and silicon, which are used to derive a lithology indicator. Westaway et al. do not describe converting either the capture relative elemental yields or the inelastic relative elemental yields to absolute elemental yields.
In U.S. Pat. No. 4,810,876, Wraight et al. disclose an indirect method for converting relative elemental yields, from activation and capture gamma ray spectra, to absolute elemental yields. Although this approach provides improved results relative to earlier conversion methods, it is desirable to provide a more straightforward method of converting relative yields to elemental concentrations (absolute yields) and for deriving the volumetric fractions of the constituents of an unknown formation. It is desirable also to provide a technique which overcomes the aforementioned difficulties associated with deriving elemental concentrations from capture gamma ray spectra and which, at the same time, affords elemental concentrations from inelastic gamma ray spectra.